1. Field of the Invention
The present invention relates to a reflection type optical element using a diffraction grating, and more particularly, to an reflection type diffraction element that is suitably used for an immersion diffraction element to be used for a spectrophotometric analyzer in fields of astronomy, biology, medicine, and the like, and to a method of manufacturing a diffraction element.
2. Description of the Related Art
In recent years, in the field of astronomy, it is expected to realize an immersion diffraction element for an infrared high dispersion spectroscopic apparatus.
FIGS. 7A and 7B are diagrams illustrating a principle of the immersion diffraction element. FIG. 7A is a schematic diagram of a conventional reflection type diffraction element, and FIG. 7B is a schematic diagram of an immersion diffraction element. As illustrated in FIG. 7B, the immersion diffraction element is a reflection type diffraction grating whose optical path is filled with a transparent medium. An optical path difference ΔL is given by 2 nL that is proportional to a refractive index n of the medium, and hence the immersion diffraction element can provide a wavelength resolution proportional to the refractive index n compared with an ordinary reflection diffraction grating whose optical path is vacuum or air (n=1) illustrated in FIG. 7A. On the contrary, in case of maintaining the wavelength resolution, a size of the apparatus can be advantageously reduced to 1/n.
In astronomy, it is desired to increase a diameter of an infrared telescope, and in this case, a diffraction element to be used for the spectroscopic apparatus is also required to be a larger size. In the spectroscopic apparatus using an ordinary reflection type diffraction element, an accompanying refrigerator and the like also become large so that huge cost and sophisticated technology are required. Therefore, it is effective to use the immersion diffraction element that can solve the problem by downsizing the spectroscope.
As a material of the immersion diffraction element described above, there is used an infrared optical element material such as cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe), germanium (Ge), or zinc selenide (ZnSe). These materials have features in that they do not have transparency to visible light but have transparency to infrared light, and have a large refractive index. However, the materials are brittle materials, and hence it is difficult to form a fine grating.
In other words, in order that the resolution reaches 30,000 in a wavelength hand, of 10 μm to 20 μm, it is necessary, for example, to finely form an echelle grating having a height of approximately 10 μm and a width of approximately 270 μm, for example, at a pitch of approximately four per millimeter on a grating surface made of cadmium zinc telluride (CdZnTe). In addition, a blazed surface of the echelle grating becomes a reflecting surface, and hence the blazed surface needs to be mirror-finished.
Conventionally, the echelle grating is manufactured by so-called machining, namely grinding through use of a grindstone, cutting through use of a diamond tool, or the like. In this case, a brittle mode machined surface inherent in a crystalline material is liable to be dominant. In order to obtain shape accuracy, dimensional accuracy, and surface roughness required for the optical element, there is a challenge to achieve processing in a ductile mode processing condition.
For instance, Japanese Patent Application Laid-Open No. 2001-246561 discloses that a ductile mode machined surface superior in surface roughness can be obtained without clogging by processing on germanium, gallium arsenide, or lithium niobate by an electrolytic in-process dressing grinding method (ELID grinding method) using a metal-bonded grindstone having an abrasive grain size #20,000.
In addition, Japanese Patent Application Laid-Open No. 2003-075622 discloses that a surface roughness of approximately 10 nmRMS can be obtained by processing on a surface (111) of Si or Ge crystal as a machined surface by a fly cutting system using a diamond tool.
Each of the above-mentioned methods of Japanese Patent Application Laid-Open No. 2001-246561 and Japanese Patent Application Laid-Open No. 2003-075622 can realize the ductile mode processing of the infrared optical element material so that good surface roughness can be obtained. However, because the material to be processed is a crystalline material and because stress is liable to be concentrated at a vertex portion of the grating during processing, the vertex of the diffraction grating is liable to a defect (chip or crack). Thus, it is difficult to completely suppress the defect.
If the vertex of the grating has a defect, light entering the grating generates scattered light at a defect portion so that diffraction efficiency is lowered, and it becomes difficult to obtain an immersion diffraction element that can satisfy the optical performance.